swapEndian.js

/**
 * Swap the endian-ness of bits (bitLen=1), bytes (bitLen=8), or whatever bitLen you want
 * 
 * Inspired by: https://stackoverflow.com/a/47668549
 */
function swapEndian(num, bitLen){
  // Takes num, transform to binary string
  let bin = num.toString(2);
  let len = bin.length;

  // Pad binary string with 0's in front if needed
  if(len % bitLen != 0){
    bin=bin.padStart(len + (bitLen - (len % bitLen)), '0');
    len=bin.length;
  }

  const result = [];
  while(len > 0){
    result.push(bin.substr((len-bitLen),bitLen));
    len -= bitLen;
  }

  let rtn = result.join('');
  return parseInt(rtn,2);
}